The present invention relates to a thermoplastic resin-based molding composition or, more particularly, to a molding composition based on a thermoplastic resin as the matrix compound with a specific filler and suitable for molding various shaped articles used in a wide variety of applications including, for example, structural parts in electric and electronic instruments, industrial machines and transportation machines such as automobiles as well as furnitures and other household commodities.
As is well known, thermoplastic resins have advantages over metallic materials in respect of their good workability, excellent corrosion resistance, lightness in weight and inexpensiveness so that they are useful as a base material for the manufacture of various shaped articles such as structural parts of instruments and machines as well as furnitures and other household commodities.
Thermoplastic resins in general, however are inferior in the mechanical properties such as tensile strength, impact strength and hardness in comparison with metallic materials and are not quite satisfactory in respect of the heat resistance and dimensional stability. Moreover, the usually advantageous feature of lightness in weight may in some cases cause a disadvantage to give a trifling impression as commercial goods.
A proposal has been made to solve the above described problems by compounding a thermoplastic resin with a metallic filler. For example, thermoplastic resins such as polypropylene, polyethylene, polyamide, poly(ethylene terephthalate), poly(butylene terephthalate) and the like are compounded with a fine powder of a metal such as zinc, copper, iron and the like as a filler to give a resin-based composite molding compound.
Shaped articles prepared from such a metal-filled molding compound, however, more or less have an unavoidable serious problem of rusting of the metal particles contained in the shaped article to cause poor appearance or degradation in the mechanical properties when the article is prolongedly kept in humid air, water, soil or other corrosive environment. When rusting occurs in the molding compound before molding, which is usually in the form of pellets, moreover, drawbacks are sometimes caused in the molding work of such a deteriorated molding compound.
Several methods have been proposed to solve this problem including a method of forming a corrosion-resistant coating layer of, for example, a rust inhibitor, metal plating and ceramic on the surface of the shaped article, a method of forming a relatively thick uniform skin layer of a rust-free resin on the surface of the shaped article and a method of wrapping the shaped article with a film of a plastic resin having a small permeability to moisture.
These methods, however, cannot provide a complete solution of the problem. For example, the corrosion-resistant coating layer formed on the surface of a shaped article has a problem of inherently low durability because the coating layer gradually falls off the surface with time. In addition, the productivity of shaped articles must decrease because the process is complicated for forming such a protective surface layer. The method of forming a skin layer is particularly defective when the shaped article has an irregular configuration or non-uniform wall thickness due to the extreme difficulty in obtaining a skin layer of uniform thickness. The productivity of course cannot be high enough. The method of wrapping with a plastic film also suffers low productivity due to the complicated and troublesome process if not to mention the limited applicability of the method.
In addition to the above described problem due to rusting of metal particles, conventional metallic filler-loaded composite molding compounds have problems in the moldability of the compound and stability of the shaped articles therefrom, especially, when the molding compound has a density in excess of 1.5 g/cm.sup.3 or, in particular, 2.0 g/cm.sup.3. The moldability of such a molding compound is poor so that the appearance of the articles shaped therefrom is sometimes not quite acceptable. Moreover, the shaped articles of such a molding compound usually have relatively low mechanical strengths or, in particular, impact strength and exhibit a large molding shrinkage sometimes with different ratios of shrinkage between directions. Therefore, it is a rather difficult matter to obtain a satisfactory shaped article of high density with good dimensional stability exhibiting no warping distortion from such a metallic filler-loaded molding compound.
Further, degradation of the matrix resin may sometimes take place in such a metallic filler-loaded composite molding compound due to the influence of the metal particles. The molding compound is also not quite satisfactory as a moldable electroconductive material because sufficiently high electroconductivity can hardly be obtained or the reproducibility of the conductivity, if obtained, is usually low.